You need to do more research. 20G would be a lousy turbo for autox. Small turbos typically have lower boost thresholds and faster transient response. Larger ones typically make more power, with peaks arriving higher in the rev range, have higher boost threshold and suffer slower transient response. A 20G isn't big in the grand scheme of things but it's much more lag than typically helps in autox.
One way to approach this is to get some rides in cars with different turbos and see which would best suit your needs. Another way is to have a look at the matrix again. Decide how much you want to invest in supporting mods, pick turbo accordingly.
Don't type things you wouldn't dare say to someone's face.
Obligatory 'build thread